1. Field of the Invention
The present invention relates generally to an apparatus for generating a parallel beam with a high flux through the appropriate arrangement of mirrors, and more particularly to an apparatus for generating a parallel beam with a high flux, in which existing optical component parts thereof are effectively arranged, so the flux of an X-ray, a neutron beam or the like is increased and the divergence of the X-ray, the neutron beam or the like is reduced.
2. Description of the Prior Art
Since visible light, an X-ray and a neutron beam allow the artificial selection of wavelengths, they are utilized to analyze structures in the fields of the atomic array of solid materials, a semiconductor, an optical element and biochemistry. As illustrated in FIG. 1, light radially propagates from a light source, so the flux of light is in inverse proportion to the square of a distance between the light source and an observer. This means that the fluxes of light are significantly reduced at the positions of a sample and a detector that are employed to analyze the structure of material. FIG. 2 is a diagram of a simple slit type X-ray reflectometer.
Further, in the cases where slits for line focusing (for instance, in reflectometers for measuring thin films) and point focusing (for instance, in four circle diffraction for measuring single crystals) are used, the fluxes of light are further reduced.
As a result, major laboratories and equipment companies continued to carry out research to increase the flux of a beam and reduce the divergence of a beam. Particularly, in the field of neutron scattering, a cold neutron source and a neutron guide are employed so as to increase the flux of a neutron beam having a certain wavelength.
FIGS. 3a to 3c are views showing a method of generating a parallel beam using a Goebel mirror (a kind of X-ray mirror) provided by Bruker Co. (Germany). FIG. 3a is a view showing the arrangement of the Goebel mirrors, FIG. 3b is a view showing the principal of generating a parallel beam using the Goebel mirrors, and FIG. 3c is a graph showing reflectance measured using the Goebel mirrors. In the case of using these Goebel mirrors, the flux of light is increased about 20 times that obtained using a simple X-ray analyzing apparatus, so the Goebel mirrors are widely utilized.
These Goebel mirrors have a hyperbolic geometry       (                                        x            2                                a            2                          -                              y            2                                b            2                              =      1        )    .Although the flux of a beam can be increased as the Goebel mirrors approach the center of a hyperbola, the Goebel mirrors cannot approach an X-ray source due to the arrangement of a beam, and it is difficult to generate a completely focused line beam (linear beam<0.1 mm).
Further, in an atomic reactor generating neutrons, it is difficult for neutron mirrors to approach a position near the center of a hyperbola (a neutron source) and the sizes of mirrors must be increased to prevent the divergence of a beam in the case of reflecting the beam using mirrors positioned at a distance, so there is no advantage in terms of increasing the flux of light.
Another method is implemented using a capillary tube as shown in FIGS. 4a and 4f. This method can be applied to both a neutron beam and an X-ray because a beam dispersing at a wide angle can be focused and a parallel beam can be easily generated, and this method can be used in a limited space because the diameter of the capillary tube can be reduced. However, the intensity of a neutron beam or an X-ray is reduced due to multiple reflection as the neutron beam or the X-ray passes through the capillary tube, and the number of rays is dependent on the thickness of the capillary tube. So, the efficiency of the method is only 10˜50%. The minimization of the diameter (about 5˜50 micrometers) and thickness of the capillary tube is a principal factor in determining the efficiency of use of a limited space. Meanwhile, X-ray Optical System Inc. developed and is selling such capillary tubes, but these capillary tubes are very expensive.
A third method is implemented by focusing a beam and generating a parallel beam in such a way as to adjust the sizes of lattices by replacing crystal lattices with graded impurities (Si→Ge), which requires complete control during the growing of a crystal. There is a report that indicates the resolution of such a problem (A. Erko, F. Schaerfers, W. Gudat, N. V. Abrosimov, S. N. Rossolenko, V. Alex, W. Schroedoer, Nucl. Instr. Meth. Phys. Res. A374 (1996) 408). However, technical difficulties still remain, and a beam diffracted by a crystal is difficult to use because it has a weak flux compared to a reflected beam.
FIGS. 5a to 5e show methods of focusing beams and forming parallel beams through the use of graded crystals. FIGS. 5a and 5b show mirrors simply using graded crystals, which are a wide-angle mirror and a focusing mirror, respectively. FIGS. 5c to 5e show devices using asymmetric graded crystals, which are a narrow beam conditioner, a symmetric collimator, and an ultimate collimator, respectively. See P. Petrashen A. Erko, Graded SiGe crystals as X-ray collimators, Nuclear Instruments and Method in Physics research A467-468 (2001) 358-361.
When these methods are used, the sizes of gratings are changed through the growth of crystals. Accordingly, it is not necessary to bend crystals using physical force because a crystal itself functions as a focusing bender, and parallel beams can be formed by cutting crystals in desired directions and therefore adjusting incident angles.